1. Field of the Invention
One of the important functions that the keel on a sailboat performs is to impede the movement of the boat sideways through the water (i.e., impede "leeway") while facilitating forward motion of the boat through the water (i.e., allowing "headway"). The control of leeway is particularly important when the sailboat is sailing "to weather" that is when the boat is sailing upwind at an angle of typically 35 to 45 degrees from the direction from which the wind is blowing.
FIG. 1 depicts a sailboat 1 sailing upwind on starboard tack at an angle 2 of approximately 45 degrees from the direction 3 from which the wind is blowing. Because of the force of the wind on the sails of the boat, the movement of the boat through the water, as represented by vector 4, normally is not quite in line with the center line 5 of the boat. The force of the wind instead causes the motion of the boat through the water to have a component represented by vector 6 ("leeway") at right angles to the center line of the boat and a component 7 ("headway") along the center line of the boat.
FIGS. 2a and b depict a sailboat 8 of the prior art having a hull 9 to which is attached a keel 10 and a rudder 11. The longitudinal dimension of keel 10 is aligned with the center line of the boat so as to impede leeway and facilitate headway.
In the prior art, keel 10 normally has a cross section which is symmetrical with respect to the center line of the boat. Because the keel moves through the water at a slight angle to the axis of symmetry, the keel generates "lift" in the direction opposite to that of the vector 6 in FIG. 1 and it is this "lift" that impedes leeway. The keel also generates drag, which drag resists the forward motion or headway of the yacht. A measure of the performance of the keel in resisting leeway while allowing headway is the ratio of lift to drag, i.e., the lift-drag ratio.
2. Description of the Prior Art
As depicted in FIG. 2a, a few racing sailboats, most notably some of the prior contenders for the America's Cup Competition, added a trim tab (or in aerodynamic nomenclature, a trailing edge flap) to the trailing edge 7 of keel 10. Trim tab 5 is attached to trailing edge 7 of keel 10 in a manner such that tab 5 can be rotated about an axis located adjacent to trailing edge 7 of the keel so as to move trailing edge 6 of the tab from side to side of the centerline of the boat.
In such prior art boats, a primary purpose of the trim tab was to act as a second rudder, which rudder is then readjusted from time to time to various fixed positions so as to "trim" the amount of the steering force that must be provided by rudder 11 to a small value. A secondary purpose for adjusting the position of the trim tab to one side or the other of the centerline was to alter the effective shape of the combination of the keel and trim tab to an asymmetrical shape such that the asymmetrical combination of the keel and trim tab, operating in cooperation with the hull of the boat, would exhibit a higher lift to drag ratio than would be exhibited by a symmetrical keel and hull. An example is depicted in FIG. 1 which indicates the starboard side 12 and the port side 13 of the sailboat. In the circumstances depicted in FIG. 1, where the wind is blowing on the starboard side of the boat, the trim tab 7 would be rotated a few degrees from the centerline so as to place the trailing edge 6 on the port side of the boat.
A disadvantage of the prior art keel described above, is that trim tab 5 must be rotated several degrees in order to obtain a small change in the effective angle of attack of the keel and trim tab assembly relative to the center line of the boat. Such a large rotation of tab 5 causes a reduction in the lift to drag ratio of the combination of the tab and keel relative to that produced by the present invention.
FIGS. 3a and b depict a different development in the keels used in the prior art by America's Cup competitors. Most notably, a keel similar to that depicted in FIGS. 3a and b was used by the successful Australian contender in 1982. In FIGS. 3a and b, keel 13 has a vertically oriented body and, in addition, has a pair of wing like appendages 15 extending sideways from the bottom of the vertical portion 14 of the keel and angling downwards slightly from the horizontal forming a wing like shape extending from the lower portion of the keel. The appendages 15 serve two purposes. First, when the boat is sailing upwind, appendages 15 act as a barrier or cap at the bottom edge of the keel that obstructs the formation of vortex flow around the bottom edge of the vertical portion 14 of the keel and thus avoids a reduction in the lift generated by the keel that otherwise would result from such vortex flow. As a consequence, the appendages can improve the lift to drag ratio of the hull and keel combination. Second, as depicted in FIG. 4, when the boat is heeled while sailing upwind, the leeward appendage 15 is positioned more deeply in the water, at depth 16, than would a conventional keel (depth 17) of equal depth when not heeled. Thus, when the boat is heeled over, the additional effective depth of the keel also may operate to improve the effective lift to drag ratio of the combination of the hull and keel.